Chapter 19: Photosynthesis & Light Reactions

Chloroplast

Organelle in plants where photosynthesis occurs, contains thylakoid membranes, grana, & stroma, which facilitate light absorbtion & energy conversion.

Thylakoid Space

Fluid-filled interior of thylakoid vesicles that play key role in proton accumulation during light reactions of photosynthesis (contributing to Proton Motive Force for ATP synthesis)

Thylakoid Membrane

Membrane where light-dependent reactions occur, while housing PSI, PSII , ATP synthase, cytochrome bf complex, & electron carriers, that are used for capturing & turning light energy into chemical energy.

Stroma

Fluid-filled region inside the chloroplast where the Calvin Cycle occurs, as well as where ATP & NADPH are generated in light reactions

O2 Evolving Complex

Protein complex in PSII responsible for oxidizing water molecules, releasing O2, protons (H+), & electrons. (cycles through 5 oxidation states, splitting 2 H2Os into O2 & 4H+)

Z Scheme

Diagram showing the flow of electrons in the light reactions of photosynthesis (Electrons flow from water to NADP+, through PSII, cytochrome bf, & PSI in a zigzag shape, resulting in ATP & NADPH)

Quantum Yield

Efficiency of photosynthesis, measured as the amount of O2 produced per photon absorbed (8 photons for 1 O2, reduce to 2 NADP+, & generating 2.667 ATPs)

Cytochrome bf Complex

Protein complex in ETC that transfers electrons from Plastoquinone to Plastocyanin while pumping protons into the thylakoid space, creating proton gradient for ATP synthesis

Grana

Stacks of thylakoid membrane that increase the surface area available for light absorption & house the light-harvesting complexes, reaction centers, & electron transport proteins for photosynthesis

Lamellae

Unstacked thylakoid membrane that connect grana stacks, while facilitating structural organization & electron transfer between regions of the chloroplast

Cyclic Photophosphorylation

Process where electrons from PSI cycle back to Cytochrome bf, generating ATP without producing NADPH or O2 (useful when cells need more ATP than NADPH)

Reaction Center

Core of PS, containing special chlorophyll molecules (P680 in PSI & P700 in PSII). Also where photoinduced charge separation occurs, leading to Z Scheme.

Antenna Chlorophyll

Collection of chlorophyll & accessory pigments in light-harvesting complexes that absorb & transfer light energy to Reaction Centers

Photophosphorylation

Process of ATP synthesis driven by light energy. Can produce ATP, NADPH, & O2 in non-cyclic & only ATP in cyclic.

Photosystem II (PSII)

1st Photosystem in light reactions, absorbing light at P680. Catalyzes the splitting of water, producing O2, H+, & high-energy electrons that enter ETC.

Photosystem I (PSI)

2nd Photosystem, absorbing light at P700. It receives electrons from cytochrome bf via PC & transfers them to ferredoxin, which reduced NADP+ to NADPH.

Chlorophyll

Main photosynthetic pigment, responsible for light absorption. It has a porphyrin ring with a central Mg2+ ion, & a long phytol tail.

Accessory Pigments

Pigments that expand the range of light absorption beyond chlorophyll & they help protect against photooxidative damage.

Plastocyanin (PC)

Copper-containing protein that transports electrons from cytochrome bf to PSI

Plastoquinone (PQ)

Lipid-soluble molecule that carries electrons from PSII to cytochrome bf, & transfers protons into the thylakoid lumen.

Photoinduced Charge Separation

Primary event in photosynthesis, where an excited chlorophyll molecule donates an electron to ETC, starting ATP & NADPH production.

P700

Reaction Center chlorophyll of PSI, absorbing light at 700 nm, donating electrons to reduce NADP+

P680

Reaction Center chlorophyll of PSII, absorbing light at 680 nm & removes electrons from water, producing O2

Proton Motive Force

Electrochemical gradient formed by proton accumulation in the thylakoid space, driving ATP synthesis as proteins flow back into the stoma via ATP synthase

Light-Harvesting Complex

Network of chlorophylls, carotenoids, & proteins that capture light energy & transfer it to the reaction center for photosynthesis.